1. Field of the Invention
The present invention relates to a variable electronic component, and in particular to a chip-type variable resistor.
2. Description of the Related Art
Typically, a chip-type variable resistor (semifixed variable resistor) includes a ceramic insulating substrate (formed with a center hole vertically extending through the substrate), a metallic terminal plate (having a central axis and fitted into the center hole from below), and a metallic rotor provided on the upper surface of the substrate in a rotatable manner about the axis.
The rotor is a twofold type or a threefold type. The twofold-type rotor, for example, includes an upwardly open cup-shaped member (having an upper end formed with an outward flange), and an upper plate member (arranged on the upper side of the flange) or lower plate member (arranged on the lower side of the flange). The flange of the cup-shaped member and the upper or lower plate member are integrally connected to each other via a hinge which is provided at an appropriate position on the periphery of the flange.
In either type, the rotor can be produced in the following manner. First, an intermediate product is prepared by sheet metal processing with respect to a material metal sheet. This intermediate product, in an opened-up state, includes a cup-shaped member and an upper or lower plate member, which are formed integral with each other via a hinge portion. Then, the intermediate product is folded at the hinge portion, thereby causing the cup-shaped member and the upper or lower plate member to be vertically overlapped vertically. Thus, the required rotor is obtained. When use is made of an upper plate member, an engagement hole for engagement of a screw driver is made in the upper plate. When use is made of a lower plate member, a hole or a groove for engagement of a screw driver is made in the cup-shaped member.
The upper surface of the insulating substrate is formed with an U-shaped resistive layer, and the ends of the resistive layer are connected to side electrodes formed on a side surface of the substrate. The above-mentioned terminal plate is proved with an upward electrode located opposite to the side electrodes with respect to the rotor (in other words, the upward electrode and the side electrodes are symmetrical in position with respect to the rotor). The upward electrode and the two side electrodes are soldered to a printed circuit board, for example.
The rotor need be provided with a contact member to come into sliding contact with the resistive layer. When the upper plate member is used, the contact member is provided at the cup-shaped member. When the lower plate member is used, the contact member is provided at the lower plate member. In either case, the contact member is located opposite to the hinge portion with respect to the axis of the rotor, and held in resilient contact with the resistive layer.
The above-described twofold-type rotor is more advantageous than the threefold-type rotor in that the former can be smaller in height than the latter, and that the material cost and the production cost can be saved.
However, according to the prior art, the cup-shaped member is connected to the upper or lower plate member via only the hinge portion, which is rather narrow. In addition, the contact member is urged strongly to the resistive layer. Thus, as the rotor is rotated for resistance adjustment, the friction drag between the contact member and the resistive layer can become stronger than the bending strength of the hinge portion, whereby the upper or lower plate member may be deformed or broken. Another problem with the prior art is a failure of proper resistance adjustment, which can happen when the contact member is not rotated properly, that is, in conformity to the rotation of a screw driver used for resistance adjustment.
In this connection, JP-A-H11-233316 teaches a structure for preventing deformation and breakage of the lower plate member. Specifically, a tongue-like bent piece is provided at either the flange of a cup-shaped member or the lower plate member, while a cutout is provided at the other of the two members. The bent piece comes into engagement with the cutout for prevention of the lower plate member's deformation or breakage.
As noted above, the rotor is produced by folding an intermediate product at its hinge portion, so that the cup-shaped member and the upper or lower plate member are vertically overlapped. However, according to the teachings of JP-A-H11-233316, in forming the tongue-like bent piece, a material metal sheet is first subjected to a punching process to prepare an unbent tongue, and then to press working to bend the tongue. Unfavorably, such a multi-stage procedure results in an increased production cost. Further, the formation of the engagement cutout may reduce the mechanical strength of the member formed with the cutout.